Lean premixed combustion systems have been deployed on land based gas turbine engines to reduce emissions, such as NOx and CO. These systems have been successful and, in some cases, produce emission levels that are at the lower limits of measurement capabilities, approximately 1 to 3 parts per million (ppm) of NOx and CO. Although these systems are a great benefit from a standpoint of emission production, the operational envelope of the systems is substantially reduced when compared to more conventional combustion systems. As a consequence, the control of fuel conditions, distribution and injection into the combustion zones has become a critical operating parameter and requires frequent adjustment, when ambient atmospheric conditions, such as temperature, humidity and pressure, change. The re-adjustment of the combustion fuel conditions, distribution and injection is termed tuning.
Controlled operation of a combustion system generally employs a manual setting of the operational control elements of a combustor to yield an average operational condition. These settings may be input through a controller, which as used herein shall refer to any device used to control the operation of a system. Examples include a Distributed Control System (DCS), a fuel turbine controller, a programmable logical controller (PLC), a stand-alone computer with communication to another controller and/or directly to a system.
These settings are satisfactory at the time of the setup, but conditions may change and cause unacceptable operation in a matter of hours or days. Tuning issues are any situation whereby any operational parameters of a system are in excess of acceptable limits. Examples include emissions excursion outside of allowable limits, combustor dynamics excursion outside of allowable limits, or any other tuning event requiring adjustment of a turbine's operational control elements. Other approaches use a formula to predict emissions based on gas turbine's operating settings and select a set point for fuel distribution and/or overall machine fuel/air ratio, without modifying other control elements, such as fuel gas temperature. Still other approaches employ a remote connection to the site by tuning experts, that will periodically readjust the tune, from the remote location. These approaches do not allow for continuous timely variation, do not comprehensively take advantage of actual dynamics and emission data or do not modify fuel distribution, fuel temperature and/or other turbine control elements.
Another variable that impacts the lean premixed combustion system is fuel composition. Sufficient variation in fuel composition will cause a change in the heat release of the lean premixed combustion system. Such change may lead to emissions excursions, unstable combustion processes, or even blow out of the combustion system.
In recent years, over-capacity of power generation, even that using F-class firing temperature gas turbines, has resulted in much of the installed gas turbine fleet running in a cyclic mode versus baseload operation. This means that many gas turbine operators are forced to shut their equipment down overnight, when power prices are so low that the losses incurred running overnight far outweigh the costs of starting the equipment every morning. This operation process has an impact on the maintenance of the equipment as each stop/start cycle causes a resulting load cycle on the equipment.
To combat this situation, gas turbine operators are investigating ways of running their equipment overnight while incurring the smallest economic loss possible. One viable solution is to lower the minimum load a gas turbine can achieve while still maintaining acceptable emissions levels. This method of operation is commonly referred to as “Turndown.”
“Turndown” has been used within the power generation industry for many years. As such, nothing directly related to this mode of operation is included as part of this patent. What is novel is the approach used by the ECOMAX™ tuning controller to tune the combustion system while in turndown, as well as the method incorporated within ECOMAX™ to mitigate detrimental effects on the combined-cycle heat recovery steam generator (HRSG) caused by low steam flows and high gas turbine exhaust temperatures. The system is also applicable to simple cycle operation; however, most simple cycle systems are applied to peak power generation and have a desirable shut-down process in the operating plan.
Often, as gas turbine loads are reduced, HRSG steam flows reduce while gas turbine exhaust temperatures rise. This combination, in conjunction with inadequate intra-stage attemperation flow capacity, often results in excessively high HRSG outlet steam temperatures (steam turbine inlet steam temperatures). In many cases these steam temperatures approach material limitations and can lead to pre-mature component failure. On the other extreme, steam conditioning/attemperation systems with adequate condensate flows can provide enough condensate to keep the superheat outlet temperature within specifications at the point of entrance into a steam turbine; however, localized over-attemperation can occur. This over-attemperation often leads to condensate impacting directly on steam piping downstream of the attemperator, causing excessive thermal fatigue in the piping sections immediately downstream of desuperheaters/attemperators.
To date efforts have focused on manually (if at all) modifying a gas turbine's fuel-to-air (f/a) ratio to keep the HRSG design constraints satisfied. However, factors such as ambient temperature changes, turbine component degradation, etc., can necessitate periodic manipulation of the gas turbine's f/a ratio, at low loads, to ensure acceptable HRSG inlet conditions. Automated manipulation of the f/a ratio of a gas turbine utilizing real-time HRSG operational information, as well as real-time gas turbine operational information, provides an efficient means to maximize HRSG component life.
It is understood that manipulation of a gas turbine's fuel-air ratio will directly affect the engine's “tune”, and as such any approach to accomplish this must be accompanied by another automated turbine control scheme to “re-tune” the turbine as-needed.
Mis-operation of the combustion system manifests itself in augmented pressure pulsations or increasing of combustion dynamics Pulsations can have sufficient force to destroy the combustion system and dramatically reduce the life of combustion hardware. Additionally, improper tuning of the combustion system can lead to emission excursions and violate emission permits. Therefore, a means to maintain the stability of the lean premixed combustion systems, on a regular or periodic basis, within the proper operating envelope, is of great value and interest to the industry. Additionally, a system that operates by utilizing near real-time data, taken from the turbine and HRSG sensors, would have significant value to coordinate modulation of fuel composition, fuel distribution, fuel gas inlet temperature, and/or overall machine f/a ratio (HRSG inlet temperature and airflow).